Diode ladder filter
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The ladder filter is famous for its use in moog synthesisers. It uses a ladder of diodes linked with capacitors to create a voltage controlled filter - this one allows you to replace one or all of them with natural semiconductors. This is quite a tricky one to use, but it sounds great when it does - capable of unstable bubbling and plenty of unexpected squeals at high resonance.
Arsenic Labyrinth was made using the first version of this plus a squarewave module with a volca modular. Part of the dramatic sounds this makes are down to using natural semiconductors in a feedback circuit, so their odd characteristics and noisiness compounds on itself in the signal, something taken further by the chaos triangle and chaos relaxation modules.
The schematic and PCB has space for diodes in the feedback amplification (D7 and 8) but I never found they made any difference to the resonance, so I've not actually soldered them in yet. Partly this will be down to the voltage I'm running it at - 9V, it won't actually run at much lower voltages than this presumably down to the voltage drop across all the diodes.
Most of the circuit is concerned with creating symmetric DC offsets to the signal before feeding it into the filter, this was based on a design by Moritz Klein which I liked because of it's relative simplicity.
In a live setting I like to use it with a antique radio crystal detector with galena. This is because in combination with another already tricky to run crystal noise source, you multiply your chances of not getting any sound - and this is more likely to work, and stay running once set in position.
Licenced under the CERN Open Hardware Licence Version 2 – Strongly Reciprocal
Inputs
- Six diodes via PCB socket connector
- Cutoff CV
Controls
- Cutoff
- Resonance
Outputs
- Audio signal
Schematic
PCB layout
